The present invention relates to a ceramic material which can be used for making the emission cathode of an electron gun of the low temperature direct heating type.
Cathodes of electron guns have to produce a high flow rate and generally function by thermoelectronic action. The materials used must satisfy a number of requirements, i.e.:
supply a high current at the operating temperature, which presupposes a high emissivity and a low work function,
have a low evaporation rate at the operating temperature,
have a sufficiently good hot mechanical property for the emission cathode to have a good geometrical stability so that the gun has very constant characteristics.
Among the materials which meet these requirements, there are metals (tantalum, tungsten) which are generally used at temperatures of about 2450.degree. C. (giving the saturation current of 1A /cm.sup.-2 for tantalum). In 1951 Lafferty (J. of Appl. Phys. 22, No. 3, pp.299-309) showed that certain borides of rare earth could also be used. An example is lanthanum hexaboride which has an emissivity of 29A/cm.sup.-2 /.degree.K..sup.-1 and a work function of 2.66e.v, an emission of 1A/cm.sup.-2 being obtained at 1700.degree. K.
Metal cathodes can be heated indirectly (by electron bombardment) or directly by the Joule effect. In the latter case, the metals are used as very thin sections (approximately 1/10 mm) in sheet or ribbon form and this makes it necessary for them to have a sufficiently high resistivity to permit their correct heating.
Rare earth borides cannot be obtained for such a form, because they are essentially produced by fritting. Hitherto, they have been produced in pellet form and have been heated indirectly.
A cathode is already known which uses indirect heating of a rare earth boride by contact with a pyrrolitic graphite resistor. This possibility is interesting but suffers from the disadvantage of requiring the use of minuscule pieces of graphite, which may prove difficult to use.
Moreover, the thermal and electrical contact between resistor and cathode can deteriorate during successive disassembly and reassembly operations. Nevertheless, this method has the advantage of permitting the use of solid hexaboride without electron bombardment being necessary.
Thus, high power industrial electron guns, used particularly for welding purposes, require the following properties from their cathode:
a high geometrical stability, so that solid materials are preferred,
a minimum thermal dissipation which prevents the appearance of undesired expansion of certain parts of the gun, so that materials with a low emission temperature are preferred,
possibility of direct heating by Joule effect permitting a simplification of the generators associated with the guns and their supply cables.
In connection with the first two points, rare earth borides are preferred against metals. However, the third point favours metals due to the low resistivity of rare earth borides (15.mu..OMEGA. at 20.degree. C.) associated with the impossibility of using them industrially, other than in the form of fritted products. For all these reasons, it has not hitherto proved possible to industrially use rare earth borides for producing emission cathodes of electron guns.